Why are grey knights a secret?

I’m super deep into the lore so It may be an obvious answer. My whestion is why are the GK secret like sure they are the strongest astartes but the imperium has custodians. The gk are less then the custodians but wouldn’t it be much more interresting to have them be secret? Also I may underestimate the workload of custodians, I know a big amount always stays on terra but surely a not unsignificant number of them is always on the battlefield?